I will soon be getting a 60D with a Canon 15-85 F3.5 lens. Since the lens doesn't have a small aperture will a ND filter allow me to take those beautiful blurred water photos I see? If so, what strength of filter, 2X, 4X or? Are ND filters all the same or are some brands better than others? Thanks.

The f/number stated on the lens (3.5-5.6 in this case) indicate the "maximum" aperture available on the lens at either end of the focal length range... that is to say, that is the widest open the lens will go. All lenses stop-down from what their stated maximum (except for reflex-mirror designs that are usually fixed at f/8, but I digress) so you should be able to stop down at 15mm to about f/22 and at 85mm to about f/32, which will be more than suitable to get a long shutter speed and blur water.

ND filters would help if you'd like to shoot more wide-open (at lower f/numbers). Generally you want to buy better, more expensive filters that won't degrade your optical performance but do your homework first. I know Hoya filters are often recommended but you're going to be paying a premium. It's definitely worth it though, as the more you pay the more you generally get in terms of performace - less flare, less vignetting, less ghosting, better contrast, etc.

Strengths of the filters... Depends on how much you want to/can stop down for the environment you're shooting in, but I'd probably go with a 3-stop ND filter as a starting point, or if you want to shoot with wider apertures go for more stops. A more-dense ND (5 or more stops) helps because when your f/numbers go up you get diffractive softening...

Stopping down on your lens will slow the shutter speed. However, there will come a point where image quality will start to suffer. Generally F11-F16 will be the smallest sweet spot you can go to without diffraction effecting the final image too greatly.

Your next option, to slow things further, wil be to alter your ISO. Dropping your ISO further will slow your shutter by a further stop (for example my Nikon D90 has a base ISO setting of ISO200, but I can push this to ISO100 if required).

So depending on light levels, you may be able to drop your shutter speed suitably to blur water sufficiently. However, if the light is brighter that is when a ND filter comes into play. A 2 or 3 stop ND filter could be sufficient, and if you have a circular polarizer than if that is stacked it can add a further 1-2 stops of light suppression if rotated correctly.

Beware of stacking filters though, particularly if they are cheaper. This can degrade image quality again.

Further down the scale you can get 10 stop ND filters, which are pretty extreme. They take a bit of getting used to and block so much light that shots in daylight can be taken over minutes rather than 10ths of a second. They can also introduce a colour cast, so need colour correction in pp.

The next thing to think of, if you are investing in filters, is thread size. Obviously the price increases as you get larger thread size filters. However if you want to future proof yourself then maybe buying good 77mm filters with step up rings from your current lens' filter size may be a good idea in the long run. That way, if you buy another lens, you can just buy another step up ring if required.

I wrote a post on the subject recently on this forum - I'll find it and add the link.

Use high quality filters only, there's no point in buying an expensive high quality lens if you'll be using a cheap filter that you found on the street (I'm serious, a few weeks ago I found a UV filter on the sidewalk! ). I generally use B&W filters, but there are many other great brands, I just happen to prefer them.

Also remember that super-low ISOs and super-closed apertures can degrade the image quality, as high ISOs and open apertures would do. Try not to close the lens too much, I generally keep it f/11 or below, usually with an ISO of 200.